Injection molding is one of the dominant forms of manufacturing in the world today. However, a problem with uniformity of product plagues this process of injecting a volume, or shot, of melted plastic into a mold. Uniformity is sacrificed due to the inability to perfectly control the volume of material injected into the mold. This imperfection is due primarily to failure of a non-return valve, found on all injection molding machines, to properly seal against back flow of material during the injection step.
A review of the prior art illustrates two primary methods of sealing against this back flow of material during the injection step: a ring-type shut off valve or a ball-type check valve. With either method, as the injection ram strokes forward, a ball is forced into a hole that it cannot go through or a tapered ring abuts against another ring with a complementary taper. There are various alterations on these devices that either control the length of the stroke or size of the inlet.
A non-return valve is disclosed in U.S. Pat. No. 4,105,147 to Stubbe. In Stubbe, the outer cylindrical periphery of the moveable valve has axially extending circumferentially spaced grooves. On the forward stroke of the feed screw, the grooves are moved out of alignment with the grooves in a fixed part when the screw is moved forward by the piston on the cylinder. The pressure of the plastic in the cavity forces the moveable valve part forward of the fixed valve part. The projection moves into the radial groove, thus turning the moveable valve part and moving the grooves out of alignment with the valve grooves. A surface on the moveable valve part seats on a surface of the fixed valve part stopping the flow of plastic material. In sum, on the filling end of the valve, when the screw starts its rotation and pressure is developed the ring is pushed in the downstream direction rather than upstream by the injection force. When it is forced downstream, the area downstream from the shut off device is filled at some set pressure, for example, 500 psi, that forces the screw backwards to its position to begin the next cycle. The next cycle begins on the forward stroke of the hydraulic system moving the screw forward again closing the ring, forcing the material trapped downstream into the injection mold.
The sealing surfaces in either a ring-type or ball-type valve can become contaminated with particles which prevent a perfect seal. This allows for material migration back through the valve rather than forward into the mold. This lost volume of material will cause an imperfect product from the mold. To detect these imperfect products requires significant inspection costs or inconvenience for the ultimate user of the product. To improve quality, many manufacturers have implemented statistical process control (SPC) which requires that all supplied product be identical, eliminating the need for inspection. Therefore, a need exists for a non-return valve which never fails to furnish the same shot size regardless of wear or contamination. This valve should be designed to allow its incorporation into existing injection molding machines or any other device which utilizes a non-return valve. Furthermore, this valve should be designed so that the seal can never be impaired by particles. Thus, each time the valve seals, the sealing action should shear and clear away any such particles.